You can document your schemas in an XML syntax with the 'annotation' element.
It has two element children:
xs:appinfo - typically for addition application information (e.g. relational tables, constraint rules, java method mappings).
xs:documentation - intended to contain the human-readable documentation for the construct.
You can put anything inside these elements (e.g. XHTML).
Example:
<xs:annotation> <xs:appinfo><java:method name="setBingo"/></appinfo> <xs:documentation xmlns="http://www.w3.org/1999/xhtml"> <p>This element represents a bingo game.</p> </xs:documentation> </xs:annotation>